A known image forming apparatus includes a plurality of light sources, a plurality of light-source drivers, a plurality of photosensitive members, and a controller. The light sources each include two light-emitting portions and a single light detector. The light-source drivers drive the respective light sources. The photosensitive members are provided for the respective light sources. The controller causes each of the light-source drivers to execute a light-quantity adjusting operation and an exposing operation for a corresponding one of the light sources. In the light-quantity adjusting operation, a light-source driver causes a light-emitting portion to emit light therefrom and adjust a light emission quantity of the light-emitting portion based on output from a light detector. In the exposing operation, a light-source driver causes a light-emitting portion to emit light therefrom based on image data to expose a photosensitive member to the light.